It is quite common in axial piston units to utilize a hardened steel valve plate secured to the housing of a hydraulic pump or motor of the axial piston type. The valve plate has a plurality of valve plate ports extending therethrough to provide fluid communication between the cylinder ports of a rotating cylinder block located adjacent the front face of the valve plate with hydraulic unit inlet and outlet ports located behind the valve plate. Such valve plates are relatively thick and quite often have grooves, sometimes referred to as "fishtails", extending opposite the cylinder block rotation or in a leading direction from the first valve plate opening which is connected to either the inlet or outlet port. Such construction is taught by Moon Jr. U.S. Pat. No. 3,585,901 issued June 22, 1971. The leading grooves are provided for the purpose of gradually increasing fluid communication between the cylinder ports and the respective inlet or outlet port of the hydraulic unit in a manner which decreases hydraulic shock so as to reduce both noise and cavitation. Such grooves are quite difficult and expensive to machine since they are quite small and the valve plate is of hardened steel. Furthermore, even with the complicated machining, consistent depth grooves were difficult to obtain. Prior grooves were traditionally obtained by milling or chemical etching and were generally 0.050 to 0.070 inch deep in order to obtain optimum hydraulic gradual flow increase. Furthermore the machining difficulty, optimum shaped cross sections of the grooves were impractical to obtain.
Furthermore, it is known in the prior art to have the leading groove formed by a notch milled completely through the complete depth of a relatively thick valve plate to form the fishtail. Again, typically, the valve plate was a quarter of an inch thick and thus the notch extending through the valve plate was also approximately a quarter of an inch thick. Such notch is too deep to allow gradual or an optimum increase in fluid flow and thus not extremely effective in reducing noise and cavitation damage. This is especially true since the depth of the notch is several times greater than the width of the notch.
When the fishtails are provided by the shallow grooves, the valve plate can not be reversed unless further machining is used to provide fishtail notches on the opposite side of the valve plate, thus doubling the machining necessary. While the second identified prior art valve plate having a notch extending therethrough is reversible, it is again pointed out that the notches are not effective due to their extreme depth.